If, other than a broken headstock, it's in great condition, I'm wondering why it appears to be missing a knob.
On a standard neck-through, if I recall correctly, a headstock replacement is not unreasonable. However, on a neck with a volute it may be more involved. You would lose the volute and get a standard scarf joint with front and back laminates. For a lefty looking to get into a five string Alembic for relatively little money, it might be worth checking out. If you could talk them down a good bit and if the case is included. Don't remember how much the headstock replacement would be, you would definitely check with Alembic first. On the other hand, you could just wait around until a lefty 5 string shows up with all the parts.